Rechargeable Lithium-Ion batteries are used extensively in notebook computers, cell phones and many other types of portable equipment primarily because of their relatively low cost and high energy storage capability. However, increasing demands to package more power into a given cell size are creating a strain on practical and technological limits for some geometries. This is evidenced by a marked increase in safety-related incidents where cells have has exploded, ruptured, or vented (i.e., undergone a forced expulsion of gases). The safety hazards from such incidents, although rare, include the potential for causing a fire and the risk of burns and other injury from projectiles and ejected cell contents. This problem has in fact subjected some battery vendors and equipment manufacturers to a number of well-publicized recalls.
Generally, a safety relief valve (or vent) represents one of the most important safety mechanisms on any cell. All cylindrical cells are required by law to be fitted with pressure relief devices designed to relieve excess gas pressure. The function of a safety relief valve (vent) is to keep a cell from rupturing in the unlikely event of excessive pressure buildup.
Generally, all current production of Li-Ion batteries involves the provision of venting mechanism at the top of the cell, which provides a safe manner of releasing excess internal pressure and preventing the cell from reaching excessively high pressure and rupture. This venting mechanism needs to be capable of operating at all times, especially during internal pressure buildup. Once a condition occurs that causes the pressure in a cell to rise to a dangerous level, the vent may be the only device at hand to prevent a catastrophic failure.
Historically, it has been shown that in order for a venting mechanism to work properly, not only should the vent be able to release the buildup gas pressure as fast as possible, but nothing should block the discharge channel, that is, the passage or passages through which gas must pass to reach the operating parts of the safety relief device.
Extensive pinpoint heating tests have shown that some cells are prone to rupture even before thermal runaway (i.e., a rapid increase in temperature), whence a supporting washer collapses and clogs the main venting orifice on the terminal disc (see description of FIG. 2 further below). While under normal conditions the valve “bursting disc” opens and the gas flows through the opening and exits the cell through terminal orifices, under abnormal conditions the gasket expands and collapses against the terminal and clogs the vent orifices, thereby leading to cell rupture.
In view of the foregoing, a growing and compelling need has been recognized in connection with improving upon the discussed shortcomings and disadvantages, among others.